Wood-like coating method for substrate

ABSTRACT

A protective coating method of a material such as aromatic polyurethane substrates is effected through a process that promotes coating adhesion to the substrate while providing a wood-like aesthetic. Closed cells on the surface of the aromatic polyurethane are opened via sanding techniques to promote adhesion. Once a sufficient surface is achieved, the substrate is coated using spray techniques to protect the aromatic polyurethane from ultra-violet light exposure. The wood-like appearance is achieved through embossing a wood-grain on the deformable surface and subsequently filling the grain with a contrasting agent to highlight its appearance through a reverse-roll-coating technique. Abrasion properties are achieved through a final clear top coat that allows both the contrast agent and the top coat to fully cure.

TECHNICAL FIELD

This invention provides a mechanical process by which a substrate formedof aromatic polyurethane, for example, can be coated in such a way thatit becomes an abrasive-resistant, weatherable, and wood-like substrate,particularly for the use in exterior applications.

BACKGROUND OF THE INVENTION

Numerous man-made composites and natural materials are known for myriaduses, including exterior applications. For example, aromaticpolyurethane is used in applications ranging from foam for car seats andbedding to rigid applications; the range of products made with this typeof technology is vast. Exterior use of aromatic polyurethane, as well asother materials, has been limited due to degradation and discolorationupon exposure to ultra-violet light. Over time, ultra-violet lightcauses a radical reaction in aromatic polyurethane that breaks down thearomatic portions of the polymer, causing a severe yellowing of thesubstrate. In order to make an exterior product comprised mainly ofaromatic polyurethane, or other materials susceptible to exteriorconditions, it must be coated in such a way as to prevent ultra-violetlight from reaching the substrate.

Coating composite materials for exterior use to look like wood has beencarried out by various processes. Some composite materials areencapsulated in a cap-stock; this cap-stock is generally a thermoplastictype material that can be extruded. The look of wood is generallyachieved by some type of embossing, in which the wood grain isthree-dimensionally represented. The color variation is generallyachieved by the incorporation of various colors during extrusion orpultrusion.

Coating of plastics has been achieved with a wide variety of techniques,some of which are used during the process described herein. A fewtechniques include, but are not limited to, general spray techniquessuch as high-pressure and low-pressure systems, roll-coating methods,powder coatings, hydrographics, and UV-curable coatings. Somemanufacturers rely on the three dimensional wood-grain and a solidcolor, while others will work to add a variety of colors to thesubstrate using methods such as printing to achieve a wood-likeappearance.

Described herein is a process by which aromatic polyurethane, forexample, can be coated with a fully hiding coating which has been provento protect the substrate while creating the appearance of wood bycombining general techniques in a unique process to promote adhesion,while creating the desired wood-like appearance.

SUMMARY OF THE INVENTION

In accordance with the invention presented herein, a method is providedfor coating a substrate made of, for example, aromatic polyurethane forexterior applications. The multi-step method begins with the mechanicalsanding of the substrate that, in a preferred embodiment, is mostlyaromatic polyurethane; this process creates a porous surface whichenhances mechanical adhesion. The substrate is then coated with aplural-component coating via air assisted airless spray. The substratecan be embossed prior to or after this coating process to create thewood grain appearance. The embossed pattern is then filled with acontrasting agent that is under-indexed to prevent adhesion to allsurfaces of the substrate. The excess contrasting agent is then removedusing a reverse roll-coater, which is self-cleaning and provides for thereuse of the excess contrasting agent. The substrate is then coated witha clear, plural-component coating which is over-indexed and thereby uponcuring allows for the contrasting agent and clear coating to adhere tothe substrate. This final coat adds sheen to the substrate as well asabrasion resistance to allow for high traffic on the substrate withminimal wear and tear.

In accordance with one aspect, the present invention relates to a methodfor coating a substrate, comprised of the steps of pressure embossing apattern into the substrate; base-coating the substrate with a basecoating; applying an under-indexed contrasting agent to the embossedpattern of the substrate coated with the base coating; and coating thesubstrate applied with the under-indexed contrasting agent with anover-indexed coating.

In accordance with another aspect, the present invention relates to amethod for coating an aromatic polyurethane substrate, comprising thesteps of sanding the aromatic polyurethane substrate to create porosityvia the opening of closed cells of the polyurethane; pressure embossinga pattern into the substrate; base-coating the substrate with aplural-component coating; applying an under-indexed contrasting agent tothe substrate and subsequently removing excess contrasting agent,thereby leaving contrasting agent in the embossed pattern; andfinal-coating with an over-indexed clear coating.

In accordance with still another aspect, the present invention relatesto a method for coating a substrate, comprising the steps of pressureembossing a pattern into the substrate; base-coating the substrate witha plural-component coating; applying a contrasting agent to the embossedpattern of the substrate, the contrasting agent comprising a pluralcomponent coating that does not have any catalyst, cross-linker, orhardening agent incorporated therein; and final-coating with a clearcoating, the clear coating comprising a plural component coating thathas an excess of catalyst, cross-linker, or hardening agent incorporatedtherein.

These and other features of the invention will become more apparentthrough the following summary, drawings, and description of thepreferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of a coating process of an embodiment of thepresent invention;

FIG. 2 depicts a flat-line spray booth set-up used in the presentinvention;

FIG. 3 depicts a flat-line embossing wheel used to create athree-dimensional wood grain appearance in the present invention;

FIG. 4 depicts a flat-line grain enhancer, which performs denibbing ofthe substrate, followed by spraying of a contrast agent, followed byreverse roll-coating of excess contrast agent from the surface ofsubstrate;

FIG. 5 depicts a contrast flash oven, used to prepare contrast forclear-coating; and

FIGS. 6A-6D depict four different configurations used to coat asubstrate.

DETAILED DESCRIPTION OF THE INVENTION

The following definitions describe some of the technical terms usedthroughout the description, but are not intended to be limiting.

Base coat—a base coat on the substrate; in a preferred embodiment, afully hiding, fully pigmented, plural component coating.

Contrast agent—a coating contrasting with the base coat on thesubstrate; in a preferred embodiment, a translucent un-catalyzed coatingused to enhance the look of the embossed wood grain.

Clear-Coat—a coating covering the contrast agent and the base coat onthe substrate; in a preferred embodiment, an abrasion-resistant,non-pigmented, over-indexed plural component coating.

Denibbing—a process in which nibs are removed from the finished surfaceof the substrate; for example, this can be thought of as light sandingwith a flap wheel on all surfaces, excluding the bottom portion of thesubstrate that is on the conveyor.

Emboss—to mold, deform, or carve in relief; as applicable to thismethod, a pre-molded embossing wheel is used to indent the negativeimage of wood-grain on the surface of the deformable substrate.

Flash—heating the substrate just enough to remove some of the volatilecomponents of a coating, thereby partially drying the coating.

Over-indexed coating—for example, a plural component paint that has anexcess of catalyst, cross-linker, or hardening agent incorporatedtherein.

Post-base-coat—anything occurring to the substrate after the substratehas had the base coat applied.

Pre-base-coat—anything occurring to the substrate before the substratehas had the base coat applied.

Substrate—a base material; in a preferred example, materials comprisedof more than 50% closed-cell aromatic polyurethane.

Under-indexed coating—for example, a plural component paint that doesnot have any catalyst, cross-linker, or hardening agent incorporatedtherein.

Wet on wet application—application of the coating in which a first basecoat is not cured, and a second base coat is applied directly on top ofthe wet first base coat.

Wet on dry application—application of the coating in which the firstbase coat is fully cured, and the second base coat is applied directlyon top of the fully cured first base coat, which is then cured as well.

A method has been devised, through experimentation and testing, forcoating a substrate 10, such as a substrate formed of aromaticpolyurethane, which allows for the use of the substrate in exteriorapplications. The various steps are shown in FIG. 1.

Step S1 of the method described herein is to sand the substrate in orderto remove any smooth portions of the substrate, particularly when formedof aromatic polyurethane. In a preferred embodiment, the substrate is inthe shape of lumber of any desired size, and can be shaped by any knownprocess for a particular material. The sanding process opens up theclosed cells located at the surface of the substrate creating a roughsurface. This rough surface is used to promote the mechanical adhesionof the coating to the substrate. The sanding should be sufficient enoughto promote adhesion, but not so aggressive as to affect any type offiller that may be used for the production of the substrate. Generalsanding methods as well as known mechanical sanding machines can beemployed in this step of the process.

Step S2 of the method described herein is to clean the sanded substrate10. As is apparent to those of ordinary skill in the area of coatings,all dust must be removed before coating. This is accomplished throughforced air and/or vacuuming the excess dust from the substrate usingknown methods and apparatuses.

Step S3 of the method described herein is to coat the substrate 10 witha fully hiding layer of base coat. The coating system can utilize athermal cure two-component polyurethane based system. Preferably, thesimilarities between the chemistries of the substrate and that of thebase coating promote excellent adhesion to the sanded substrate. Thebase coating can contain three components: paint, hardener and thinner.These three components can be combined in specific mix ratios in thefollowing ranges 3-6/1/1-3 (paint/hardener/thinner). All coatingsutilized are based on the same chemistries and therefore have excellentinter-coat adhesion. The base coat contains inorganic pigments thatallow the coating to be fully hiding; therefore, the base coat providesthe main UV-protection as it blocks excess UV radiation from reachingthe substrate beneath. In addition, the materials used in the inventionare not limited to those disclosed herein. Furthermore, the coatingmaterials are not limited as long as they can achieve desired results,such as abrasion resistance, UV and weather resistance, and a wood-likeappearance. The base coat is applied to the substrate using air-assistedairless spray equipment on a flat line conveyor system. The coating ofthe substrate may be accomplished in two ways.

The first coating method will be defined as wet on dry application offirst and second base coats. The first base coat is applied via airassisted airless spray methods. The spray booth 20 used to perform thisstep is shown in FIG. 2. The substrate 10 rides along a flat-lineconveyor (not shown) through a typical spray booth set-up 20 where thesubstrate receives its first base coat. An example of the spray booth 20that can be used is one fabricated to accommodate three air assistedairless spray heads 22 in order to coat the three-dimensional profilesof the substrate. The first base coat is then cross-transferred to anoven conveyer (not shown). The substrate is then fully cured bymaintaining the surface temperature of the substrate at approximately235° F. over the course of approximately 8-10 minutes through a naturalgas fired oven 30 fabricated to accommodate substrates that can reachlengths of 24 feet. The second base coat, if desired, is then appliedthe same way as the first base coat. The substrate then travels throughanother curing phase.

The second coating method will be defined as a wet on wet application ofthe two base coats. The first base coat is applied via air assistedairless spray methods in a first spray booth 20. The substrate ridesalong the flat-line conveyor through another typical spray booth 20′,and the second base coat is then applied the same way as the first basecoat. The substrate then travels through the curing oven 30 describedabove to be fully cured. This coating method only requires one curingphase, whereas the first coating method described above requires twocuring phases. This second coating method leads to a very similarappearance as the previously described method; however, the second basecoat does not remain as close to the surface as it does in the firstcoating method, leading to a slightly less uniform appearance than thatobtained in the first coating method.

The first coating method tends to give the substrate a slightly moreuniform and even coating because the second base coat can more readilyreside closer to the surface of the substrate when applied to apreviously cured first base coat. The second coating method is describedas a more economical alternative to the first coating method as a curingstep is removed from the process, and as a potential method to adddiversity to the substrate, leading to a more wood-like appearance.

Step S4 of the method described herein is to create a wood grainappearance on the surface of the substrate. Due to the deformability ofthe substrate, particularly when aromatic urethane is used, a pressureembossing wheel is used to indent the surface of the substrate, leavinga wood grain emboss. This is depicted in FIG. 3, in which emboss wheel42 of embossing machine 40 has a negative image of wood-grain and thesubstrate 10 rides along a flat-line conveyor (not shown). For thepreferred substrate, aromatic polyurethane, this process is done onlywith pressure; no heat is involved. A flame-hardened seamless steelroller 42 engraved with the negative image of a wood grain pattern 43 isused to indent the surface of the substrate, depicted in FIG. 3. As thesubstrate rides along the flat-line roller conveyor, the embossing wheel42 rotates so as to pull the substrate through the process whiledeforming the surface using pressure. The enlarged portions of FIG. 3depict a schematic of the surface of the substrate pre-emboss 10-1 andpost-emboss 10-2. The darker areas indicate the areas in which thesurface is depressed to create the wood-grain emboss pattern 12. Theembossing of the substrate can occur before or after base-coating, theformer being shown on the left side of FIG. 1 using dashed lines wherestep S4 is followed by step S3. The coating used must be flexible enoughto deform with the substrate without incurring damage or points ofpotential failure. The embossing wheel 42 has a length at least as wideas the base material.

If the substrate is base-coated prior to embossing, the embossed patternwill be minimally deeper upon starting the next process, thereby makingthe contrast agent pool more within the grain pattern, leading to aslightly darker wood grain appearance. If the substrate is embossedprior to base-coating the base coat can fill in a small portion of thegrain pattern, thereby making the contrast agent pool less within thegrain pattern, leading to a slightly lighter wood grain appearance.

Step S5 of the method described herein is to promote inter-coat adhesionand flow of a contrast agent into the emboss pattern. The base-coatedsubstrate is preferably sent through a commercially available denibbingsystem 60 that utilizes 100-220 grit flap sanders 62 to denib allsurfaces of the substrate to be coated, as shown in FIG. 4. This processcan remove nibs within the base-coated substrate to create a bettersurface for the remainder of the coating. The denibbed substrate iscleaned with forced air and vacuum (not shown) to remove particulatesthat may interfere with inter-coat adhesion.

Step S6 of the method described herein is to apply a darker translucentcoating, known as the contrast agent, to the embossed pattern of thesubstrate shown in FIG. 4. The contrast agent is a non-catalyzed coatingthat is greatly reduced in viscosity, to between 12-40 centistokes, toallow for flow of the coating into the embossed pattern. The contrastagent is lightly sprayed onto the entire surface of the base-coatedsubstrate using a single air assisted airless spray gun 72 with a largespray fan pattern to completely saturate the surface of the substrate.The substrate is transferred via conveyor (not shown) through acommercially-available, down-draft spray booth 70.

In Step S7, the excess contrast agent is removed via a self-cleaningcommercially available reverse roll-coater 80 outfitted with a 50durometer rubber roll 82 as shown in FIG. 4. A cleaning roller 84 canremove the recovered contrasting agent from the roll 82 for reuse. Thisportion of the process leaves the majority of the contrasting agent 14in the embossed pattern 12 while removing the remaining contrast agentfrom the surface of the substrate as shown in the enlarged image of thesubstrate 10-3.

In the foregoing description, the denibber 60, spray booth 70 andreverse roll-coater 80 can be positioned at a single station 50.

The substrate, in Step S8, then travels on the flat line conveyor system(not shown) through a natural gas fired contrast flash oven 90fabricated to fit over the conveyor system. The substrate spendsapproximately 5-10 seconds at 165-180° F. underneath the flash oven;this allows enough time for the contrast to be partially dried inpreparation for clear coating. The flash-off of excess solvents withinthe contrast agent allows the coating to remain within the grain patternduring clear-coating (described later). A fully wet contrast agent caneasily be washed out of the grain pattern by the clear-coat, leading toa muddled surface appearance, which does not highlight the wood grainembossed pattern.

Step S9 of the method described herein is to apply an over-indexed clearcoat to the substrate. The clear coat is based on the same chemistriesas both the base coats and contrast agent; it, however, does not containpigment and is completely transparent and therefore minimallycontributes to the UV protection of the underlying substrate. The clearcoat provides abrasion resistance as well as luster to the substrate.The application of the clear-coat goes directly on top of thepartially-dried contrast agent. The clear coat is applied via airassisted airless spray equipment 100 on a flat line conveyor system. Thecoating is then thermally cured in oven 110. The over-indexed clear coatallows the un-catalyzed contrast agent to fully cure as well.

While the foregoing description describes performing the various stepsin a particular order, that order is not limiting. Several possibleconfigurations are shown in FIGS. 6A-6D. Each process described belowbegins with sanding of the substrate to promote adhesion.

1) Pre-emboss, wet on dry application of coating, shown in FIG. 6A.

6A.1—emboss

6A.2—base coat 1

6A.3—cure oven

6A.4—base coat 2

6A.5—cure oven

6A.6—denib, contrast agent, reverse roll-coat

6A.7—contrast flash oven

6A.8—clear coat

6A.9—cure oven

2) Post-emboss, wet on dry application of coating, shown in FIG. 6B.

6B.1—base coat 1

6B.2—cure oven

6B.3—base coat 2

6B.4—cure oven

6B.5—emboss

6B.6—denib, contrast agent, reverse roll-coat

6B.7—contrast flash oven

6B.8—clear coat

6B.9—cure oven

3) Pre-emboss, wet on wet application of coating, shown in FIG. 6C.

6C.1—emboss

6C.2—base coat 1

6C.3—base coat 2

6C.4—cure oven

6C.5—denib, contrast agent, reverse roll-coat

6C.6—contrast flash oven

6C.7—clear coat

6C.8—cure oven

4) Post-emboss, wet on wet application of coating, shown in FIG. 6D.

6D.1—base coat 1

6D.2—base coat 2

6D.3—cure oven

6D.4—emboss

6D.5—denib, contrast agent, reverse roll-coat

6D.6—contrast flash oven

6D.7—clear coat

6D.8—cure oven

In each of the foregoing configurations, the conveying system is notshown. The conveying system can be comprised of a single conveyingsystem including belts and rollers, for example, for moving the basematerial through the various stations; individual conveyance systemsmodified for each station; or two or more conveyance systems, each formoving the substrate material through one or more stations comprisingthe whole system. In the event that one or more conveying mechanisms areused, a controller, such as a suitably programmed microprocessor, can beused to control the various speeds of conveyance. Differences inrequired conveying speeds between the various stations can be controlledby the controller. Alternatively, the substrate materials can be movedmanually or by other means from station to station.

Control of the various stations can be coordinated with the conveyancecontrol using the controller described above or one or more additionalcontrollers or sub-controllers.

Thus, there has been shown and described a new and useful method andsystem for creating wood-like substrates. Although this invention hasbeen exemplified for purposes of illustration and description byreference to certain specific embodiments, it will be apparent to thoseskilled in the art that various modifications, alterations, andequivalents of the illustrated examples are possible.

What is claimed is:
 1. A method for coating a substrate, comprising the steps of: pressure embossing a pattern into the substrate; base-coating the substrate with a base coating; applying an under-indexed contrasting agent to the embossed pattern of the substrate coated with the base coating; and coating the substrate applied with the under-indexed contrasting agent with an over-indexed coating.
 2. The method of claim 1, wherein the substrate is comprised of aromatic polyurethane.
 3. The method of claim 2, further comprising the step of sanding the polyurethane substrate prior to base-coating to create porosity to promote mechanical adhesion.
 4. The method of claim 1, wherein the substrate is mechanically embossed either prior to or after base-coating.
 5. The method of claim 1, wherein the applying the under-indexed contrasting agent to the embossed pattern comprises applying the contrasting agent to a surface of the substrate having the embossed pattern and removing excess contrasting agent by reverse roll-coating, thereby allowing the contrasting agent to remain in the embossed pattern.
 6. The method of claim 1, wherein the embossed pattern comprises a wood grain pattern.
 7. The method of claim 1, wherein the over-indexed coating comprises an abrasion-resistant clear coating.
 8. The method of claim 1, wherein the over-indexed coating comprises a plural component paint that has an excess of catalyst, cross-linker, or hardening agent incorporated therein, and the under-indexed contrasting agent comprises a plural component paint that does not have any catalyst, cross-linker, or hardening agent incorporated therein.
 9. The method of claim 1, wherein the base coating comprises a fully pigmented, plural component coating.
 10. A method for coating an aromatic polyurethane substrate, comprising the steps of: sanding the aromatic polyurethane substrate to create porosity via the opening of closed cells of the polyurethane; pressure embossing a pattern into the substrate; base-coating the substrate with a plural-component coating; applying an under-indexed contrasting agent to the substrate and subsequently removing excess contrasting agent, thereby leaving contrasting agent in the embossed pattern; and final-coating with an over-indexed clear coating.
 11. The method of claim 10, wherein the substrate is mechanically embossed either prior to or after base-coating.
 12. The method of claim 10, wherein the embossed pattern comprises a wood grain pattern.
 13. The method of claim 10, wherein the over-indexed coating comprises an abrasion-resistant clear coating.
 14. The method of claim 10, wherein the over-indexed coating comprises a plural component paint that has an excess of catalyst, cross-linker, or hardening agent incorporated therein, and the under-indexed contrasting agent comprises a plural component paint that does not have any catalyst, cross-linker, or hardening agent incorporated therein.
 15. The method of claim 10, wherein the base coating comprises a fully pigmented coating.
 16. A method for coating a substrate, comprising the steps of: pressure embossing a pattern into the substrate; base-coating the substrate with a plural-component coating; applying a contrasting agent to the embossed pattern of the substrate, the contrasting agent comprising a plural component coating that does not have any catalyst, cross-linker, or hardening agent incorporated therein; and final-coating with a clear coating, the clear coating comprising a plural component coating that has an excess of catalyst, cross-linker, or hardening agent incorporated therein.
 17. The method of claim 16, wherein the substrate is comprised of aromatic polyurethane.
 18. The method of claim 16, wherein the substrate is mechanically embossed either prior to or after base-coating. 